1. Field of the Invention
The present invention relates to a thermoplastic polymer foam having a multimodal cell size distribution and a water-free process for preparing said foam.
2. Description of Related Art
A foam having a multimodal cell size distribution (multimodal foam) offers performance advantages, such as greater toughness and enhanced insulating capability, over a conventional foam of the same polymer composition but having a generally uniform cell size distribution. A foam having a bimodal cell size distribution (bimodal foam) is one type of multimodal foam.
Current processes for producing multimodal foams expand a foamable polymer composition containing water. Water tends to produce corrosive acid when it reacts with halogenated fire retardants. The corrosive acid is undesirable because it can corrode process equipment. Therefore, a process for preparing a multimodal foam that does not require water is desirable. A water-free process that uses an environmentally friendly blowing agent is even more desirable.
In a first aspect, the present invention is a process for producing multimodal thermoplastic polymer foam comprising the following sequential steps: (a) dispersing a blowing agent stabilizer and a blowing agent into a heat plasticized thermoplastic polymer resin at an initial pressure to form a foamable composition; and (b) expanding said foamable polymer composition in a substantial absence of water and at a pressure less than said initial pressure to produce a multimodal thermoplastic foam. A preferred embodiment further comprises cooling the foamable composition between steps (a) and (b) and wherein step (a) further comprises applying shear to form the foamable composition. Blowing agent stabilizer is desirably present at a concentration of one to 50 weight-percent relative to polymer resin weight.
A second aspect of the present invention is a thermoplastic polymer foam comprising a thermoplastic polymer resin having large and small cells defined therein and a blowing agent stabilizer predominantly located proximate to the large cells. Desirably, the foam contains one to 50 weight-percent of a blowing agent stabilizer relative to thermoplastic polymer resin weight.
A third aspect of the present invention is an article of manufacture comprising the thermoplastic polymer foam of the second aspect. Preferably, said article is thermally insulating.
Foams of the present invention have utility as acoustical modulators, thermal insulation, and absorbent materials.